:root {
    --button-bg: #e0e0e0;
    --button-hover-bg: #d0d0d0;
    --button-active-bg: #b0b0b0;
    --display-bg: #fff;
    --calculator-bg: #fff;
    --shadow: 0 0 10px rgba(0, 0, 0, 0.1);
}

body {
    font-family: Arial, sans-serif;
    display: flex;
    justify-content: center;
    align-items: center;
    height: 100vh;
    margin: 0;
    background-color: #f4f4f4;
}

.calculator {
    background-color: var(--calculator-bg);
    border-radius: 8px;
    box-shadow: var(--shadow);
    padding: 20px;
    width: 300px;
}

#display {
    width: 100%;
    height: 50px;
    font-size: 24px;
    text-align: right;
    margin-bottom: 10px;
    padding: 10px;
    border: 1px solid #ccc;
    border-radius: 4px;
    background-color: var(--display-bg);
}

.buttons {
    display: grid;
    grid-template-columns: repeat(4, 1fr);
    grid-gap: 10px;
}

button {
    padding: 15px;
    font-size: 18px;
    border: none;
    background-color: var(--button-bg);
    cursor: pointer;
    border-radius: 4px;
    transition: background-color 0.2s;
}

button:hover {
    background-color: var(--button-hover-bg);
}

button:active {
    background-color: var(--button-active-bg);
}